The present invention relates to a method of detecting embrittlement, such as embrittlement due to passage of time, of heat-resisting steels, and more particularly to a method wherein an anode polarization curve for heat-resisting steel products is prepared to determine repassivation current values, and embrittlement of the heat-resisting steel products is detected on the basis of these repassivation current values.
Heat-resisting steels must be chemically stable at high temperatures, have good mechanical properties, and have structures which are stable at high temperatures. However, in the case of steam turbines and the like used over a long period of time, structural changes occur in the heat-resisting steel, and deterioration with time of the material is liable to become marked. Of the items of such deterioration of the material, a particularly problematic item is the so-called temper brittleness with age.
It is believed that this temper brittleness with passage of time occurs as a result of the same causes as those of temper brittleness arising in the tempering step of heat treatment by the repetition for a long period of time of heating at high temperatures during operation and cooling during suspension of operation. That is, it is believed that the temper brittleness with passage of time occurs because the impurity elements such as Sb, As, P and Sn present in steel, particularly P, segregate in the grain boundary to reduce the strength of the grain boundary. This is apparent from the fact that an electron microscope shows that the impact fractured surface of the embrittled materials is the fractured surface of the grain boundary, and from the fact that the elemental spectra of the fractured surface of the grain boundary by means of an Auger electron spectral analysis show that the impurity elements such as P segregate in the grain boundary.
When the secular temper brittleness of the heat-resisting steel products occurs, there is the possibility that it will lead to deficiencies, e.g., breakage of these steel products. Particularly, in the case of turbine rotors and the like, which must be rotated at a high velocities and withstand large centrifugal forces, a serious trouble can occur in the steam turbine. Accordingly, for quality evaluation of heat-resisting steels and trouble prevention, a variety of methods of detecting temper brittleness with age have been proposed.
Hitherto, the following methods of detecting the degree of temper brittleness of steels have been proposed. It is possible to detect temper brittleness in a steel by subjecting a sample of the steel to an impact test to determine the degree of reduction of impact value (the reduction of the value of energy absorbed). It is also possible to detect temper brittleness in terms of the shifting of a test temperature-impact value curve toward the higher temperature slide, i.e., the shifting of the transition temperature of an impact value. Further, it is possible to detect the temper brittleness by the rise in the fracture transition temperature which is defined as the temperature obtained by measuring the ratio of the ductile fracture area to the total fracture area of the fractured surface of a test piece which has been impact fractured to determine the ductile fracture percentage and taking the temperature corresponding to a ductile fracture percentage of 50% from the temperature dependency.
However, all of these prior methods of detecting temper brittleness are destructive tests wherein it is impossible to carry out such tests unless a test piece is cut off from a heat-resisting steel product which is the subject to be inspected. Therefore, it is impossible to use the steel product again after inspection, and thus it is practically impossible to detect temper brittleness during periodical inspection for the purpose of the maintenance of heat-resisting steel products.